Configuring The Root Bridge Id - Cisco AJ732A - MDS 9134 Fabric Switch Configuration Manual

Chapter 8 Configuring Rapid PVST+
S e n d f e e d b a c k t o n x 5 0 0 0 - d o c f e e d b a c k @ c i s c o . c o m
To disable Rapid PVST+ per VLAN, perform this task:
Command
switch(config)# no spanning-tree
vlan-range
Caution Do not disable spanning tree on a VLAN unless all switches and bridges in the VLAN have spanning
tree disabled. You cannot disable spanning tree on some switches and bridges in a VLAN and leave it
enabled on other switches and bridges in the VLAN. This action can have unexpected results because
switches and bridges with spanning tree enabled will have incomplete information regarding the physical
topology of the network.
Do not disable spanning tree in a VLAN without ensuring that there are no physical loops present in the
VLAN. Spanning tree serves as a safeguard against misconfigurations and cabling errors.

Configuring the Root Bridge ID

The software maintains a separate instance of STP for each active VLAN in Rapid PVST+. For each
VLAN, the switch with the lowest bridge ID becomes the root bridge for that VLAN.
To configure a VLAN instance to become the root bridge, modify the bridge priority from the default
value (32768) to a significantly lower value.
When you enter the spanning-tree vlan vlan_ID root command, the switch checks the bridge priority
of the current root bridges for each VLAN. The switch sets the bridge priority for the specified VLANs
to 24576 if this value will cause the switch to become the root for the specified VLANs. If any root
bridge for the specified VLANs has a bridge priority lower than 24576, the switch sets the bridge priority
for the specified VLANs to 4096 less than the lowest bridge priority.
The spanning-tree vlan vlan_ID root command fails if the value required to be the root bridge is less
Note
than 1.
Caution The root bridge for each instance of STP should be a backbone or distribution switch. Do not configure
an access switch as the STP primary root.
Enter the diameter keyword to specify the network diameter (that is, the maximum number of bridge
hops between any two end stations in the network). When you specify the network diameter, the software
automatically selects an optimal hello time, forward delay time, and maximum age time for a network
of that diameter, which can significantly reduce the STP convergence time. You can enter the hello-time
keyword to override the automatically calculated hello time.
With the switch configured as the root bridge, do not manually configure the hello time, forward-delay
Note
time, and maximum-age time using the spanning-tree mst hello-time, spanning-tree mst
forward-time, and spanning-tree mst max-age global configuration commands.
OL-16597-01
Purpose
Disables Rapid PVST+ on the specified VLAN; see the
following Caution for information regarding this
command.
Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
Configuring Rapid PVST+
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